When a group of African green monkeys was shipped from Uganda to Germany and then-Yugoslavia for research in 1967, nobody could’ve predicted that it would result in the infection of 31 people and the death of seven.¹ This was the result of the Marburg virus, which leads to a severe and often fatal disease called Marburg virus disease. 

It is now known that the Marburg virus doesn’t only infect primates; other wildlife plays a major role in the outbreaks and spread of this disease, too. Notably, bat colonies have been shown to act as a natural reservoir for the disease and contribute to its spread. 

Introduction

Marburg virus disease, also called Marburg haemorrhagic fever, is a rare but often fatal infectious disease. It belongs to the same family of viruses as the Ebola virus, which was responsible for the 2014-2016 outbreak of Ebola virus disease in central Africa. The World Health Organization (WHO) estimates that infection with Marburg virus has a fatality (death) rate of up to 88%. 

Whilst the specific mode of transmission and infection remains unclear, it is known that Marburg virus naturally resides in animals, and that contact with these animals has been the precipitating factor for all known outbreaks.² This means that understanding the role of wildlife in the spread of this virus is crucial in understanding how to prevent and treat the disease that it causes. 

Marburg virus disease progression

Upon exposure to infected animals or people, it takes anywhere from 2 days to 3 weeks for symptoms to show; this is called the incubation period. The symptoms start as non-specific, general symptoms of infection, such as:³

• High temperature 
• Muscle aches
• Joint pains
• General feeling of illness and discomfort (malaise)
• Nausea
• Vomitting
• Diarrhoea

As days pass, infected individuals may have the following:^3,4 

• Non-itchy rash 
• Abdominal pain
• Abdominal cramping
• Severe diarrhoea with blood
• Vomiting blood
• Slow heart rate (bradycardia)
• Vision sensitivity to light (photophobia)

In fatal cases of the disease, there are more severe symptoms, usually observed in the second week of infection:^3,5

• Bleeding out of multiple areas such as the rectum, nose, gums and vagina
• Bleeding under the skin
• Spontaneous bleeding from needle puncture sites
• Confusion or delirium 
• High heart rate (tachycardia)

Marburg virus causes increased bleeding in the body, hence, most deaths associated with it are caused by organ failure. Marburg virus also causes inflammation of the brain (encephalitis).⁶ In both cases of death and recovery, the disease causes severe damage to the liver. Survivors become immune to infection by the same virus strain after recovery, but may face long-term problems such as lasting joint and muscle aches, problems with the eyes, psychosis and social ostracisation.⁷ 

Role of wildlife in virus ecology

Marburg virus disease is a zoonotic disease, meaning that it originates in animals and is transmitted to humans. This can occur in many different ways when exposed to infected animals, from handling dead animals to eating infected meat. 

It has been estimated that 60% of infectious microbes (pathogens) that affect humans come from animals.⁸ Apart from Marburg virus, there are many known diseases with a wildlife origin, including.⁹

• Ebola virus 
• Zika virus
• COVID-19
• Yellow fever
• Rabies

Bats as natural reservoirs for Marburg virus

Although the Marburg virus was discovered following infection from African green monkeys, its natural reservoir was unclear for a long time. A natural reservoir refers to the species of animal which is the primary source of a specific pathogen. In this case, it would be the species of animal that the Marburg virus exists in naturally and depends on for survival. 

By examining local outbreaks, researchers have been able to trace the Marburg virus to bat colonies residing in caves in sub-Saharan Africa. It is generally agreed that the main species of bat linked to outbreaks of Marburg virus disease is the Egyptian rousette bat, also known as the Egyptian fruit bat or Rousettus aegyptiacus.^10,11,12 The virus, however, has also been shown to be present in other bat species, namely the greater long-fingered bat (Miniopterus inflatus) and the eloquent horseshoe bat (Rhinolophus eloquens).¹³

Marburg Virus-infected bats do not experience Marburg virus disease in the way humans do. These bats experience Marburg virus asymptomatically (without any symptoms) and transfer it by shedding, a process in which the virus is expelled from the host to infect other organisms. It is believed that the reason why infected bats don’t die from Marburg virus disease is that the immune system of the bats is adapted to cope with the infection.^14,15

Transmission from bats to humans

Infection with Marburg virus is a relatively rare occurrence in bat colonies.¹³ However, bats are social animals and come into contact with thousands of species-mates every day. This means that it is highly unlikely for a colony to ever become fully get rid of the virus. This is not an issue for the colony, as infected bats do not present symptoms or die from the disease as humans do.¹⁶ The issue arises when humans encroach on this infectious habitat. 

Humans usually come into contact with infected bats when spending prolonged periods of time in mines or caves where the bats are present. Most outbreaks have been linked to miners, as this line of work involves frequenting areas with infected bats.¹⁷ There have also been recorded cases of tourists contracting the virus after getting too close to infected Egyptian fruit bats in caves.¹⁸ 

Marburg virus is transmitted to humans after it is shed from the bat. The Marburg virus present in infected bats can be spread for up to 3 weeks after the bat is initially infected.¹⁴ There are a few mechanisms for transmission from bat to human, namely via contact with infected bat urine, saliva, blood and faeces. Furthermore, the virus can survive in the blood and organs of the bats for over a week after death.¹⁹ 

Role of other wildlife

Egyptian fruit bats are the natural source for the Marburg virus, but we already know that the virus can be spread to other wildlife, such as primates. There is not much information about these types of secondary infections causing outbreaks, but the risk persists. 

It is thought that bats can spread filoviruses (such as Marburg and Ebola) to primates when they contaminate fruits with saliva, and these fruits are subsequently eaten by primates.²⁰ Infection in primates can cause a range of responses from no symptoms to death.²¹ 

Human-to-human transmission

Once infected, humans can spread the Marburg virus to other humans with whom they come into contact. The spread of Marburg virus disease from person to person is not common and is usually associated with poor hygiene practices or very close contact with infected individuals. This is because the virus is spread between humans by the mixing of bodily fluids. The highest risk of spread is associated with the later stages of the disease, when it is most infectious.²² 

The biggest factor determining the spread of the Marburg virus is how infected individuals are handled. Poor hygiene practices in hospitals or households increase the risk of infection.⁶ To prevent further infections and deaths, hygiene practices should be maintained in both life and death. There have been cases of filoviruses spreading from traditional funeral practices, as the virus persists in the body after death.²³ 

Prevention and control measures

There are no current vaccines or treatments in use for Marburg virus disease.²⁴ The control of Marburg virus outbreaks lies in the prevention of disease contamination and spread. This includes:⁶

• Avoiding contact with bats in affected areas where possible 
• Avoiding close contact with infected individuals and animals
• Wearing appropriate personal protective equipment (PPE) when in contact with an infected individual and infected material
• Hygienic funeral practices for those who pass away from the disease

Summary

Marburg virus is responsible for causing Marburg virus disease, which is life-threatening. This disease has, in previous outbreaks, had a human death rate of up to 88%. It causes symptoms which begin as non-specific signs of an infection, including diarrhoea, vomiting and joint and muscle pains. Progression results in extreme abdominal pain, cramps and profuse bleeding from all orifices. Such severe cases usually result in death. The natural reservoir, or natural wildlife source, was identified to be the Egyptian fruit bat. Marburg virus occurs naturally in bat colonies which reside in mines and caves in sub-Saharan Africa. In infected bats, the virus does not cause any symptoms. Prolonged exposure or close encounters with infected bats are responsible for the human spread of the Marburg virus from bats because the virus is spread by the bodily fluids (saliva, blood, urine) of infected bats. The virus can also spread to primates and infect humans this way. Preventative measures for the spread of Marburg virus between humans include avoiding caves inhabited by Egyptian fruit bats and other infected species, wearing appropriate PPE when in contact with infected humans or materials and general hygiene practices.